Global emissions of mercury into the atmosphere have increased by an estimated factor of three over the past 100 years due primarily to anthropogenic releases associated with fossil fuel burning and waste incineration (particularly municipal waste). Closely associated with the increases in atmospheric mercury emissions are widespread increases in the concentrations of mercury in fish tissue, even in remote bodies of water far removed from any direct input source. Fish and other aquatic organisms have been demonstrated to be highly efficient at bioconcentrating mercury from water, and this poses a serious health risk to consumers. In addition, food chains based on aquatic organisms can lead to mercury contamination of birds and mammals. Vapor phase transport of mercury through the global environment is of great concern due to increasing energy production and industrial activity worldwide. Accordingly, monitoring of vapor phase mercury in the atmosphere and neutral mercury species dissolved in the aqueous phase of environmental systems, together with remediation of mercury contamination, will become increasingly critical for the foreseeable future.
Laboratories conducting analytical and toxicological research, concerning the presence and toxicity of mercury species, must have facilities that are practically free of vapor-phase mercury (i.e., clean at ultra-trace levels) so as to avoid false-positive results, i.e., results indicating that mercury was present in a sample, or that mercury caused a demonstrated effect, when, in fact, the mercury came from the laboratory environment. To ensure that laboratory contamination of samples by vapor-phase mercury is not a problem, a simple and effective integrative way is needed for monitoring air and water at very low levels for vapor and dissolved phase-neutral mercury species. Unfortunately, current methods are not integrative over sufficient time intervals to sequester adequate amounts of ultra-trace levels of vapor-phase mercury and to cost effectively detect episodic releases.
Currently, there are two basic approaches for sampling vapor-phase neutral mercury species: 1) using a pump to pass a known volume of air through a trap designed for collection of vapor-phase mercury (wherein, the trap typically is comprised of an inert substrate coated with gold), and 2) passive diffusive sampling into an adsorbent or gold film. With the "pump and trap" methods, the analysis can be performed in a semi-continuous manner (on site) or by a "grab sampling" approach wherein the trapped vapor-phase mercury is taken to the laboratory for analysis. These active sampling approaches suffer from the adverse consequences of potential instability (i.e., loss of adsorbed mercury through revaporization, etc.), complexity, and mechanical operation with its attendant requirement for power sources, flow rate calibration, and human oversight. Further, the presence of particulate matter in the sampled media often interferes with the sampling process. For example, as particulate matter is deposited on the sampling surface or in, or on, a prefilter media, the particulate matter acts as an additional filtration matrix with the potential of reducing flow rates through the sampling system. Further, the retained particulate matter may sorb the mercury species at a rate significantly greater than the original filter, thereby resulting in imprecise or biased mercury values.
There are two passive badge-type samplers used for vapor phase mercury. One version utilizes a thin film of gold as described in U.S. Pat. No. 3,942,219 (Brum) and in McCammon, C. S. jr. and Woodfin, J. W., An Evaluation of a Passive Monitor for Mercury Vapor, Am. Ind. Hyg. Assoc., 1997, 38, 378-386. The second uses a metal oxide solid sorbent as described in Rathije, A. O. and Marcero, D. H., Improved Hopcalite procedure for the Determination of Mercury Vapor in the Air by Flameless Atomic Absorption, Am. Ind. Hyg. Assoc. 1976, 37, 311-314. Each sampler uses a microporous membrane barrier through which mercury vapor diffuses across air filled regions to the sampling cartridge. For the gold film approach, the detection of sorbed (amalgamated) mercury is based on an electrical measure of the change in the resistivity of the gold imparted by traces of amalgamated mercury. This approach is subject to poor detection limits and the potential for interference from oxidizing agents such as chlorine gas. For the sorbent based system, potentially high background levels of mercury (and the resulting poor detection levels), resulting from the strong acid treatment required to recover the sorbed mercury prior to analysis by spectroscopic methods, present problems. No passive sampling methods for gaseous mercury dissolved in water have been demonstrated. The determination of gaseous mercury in water is accomplished using a purge and trap system employing an inert gas as the purging media and a sorbent to trap the vapor-phase mercury.
Other patents of interest in the general field include the following: U.S. Pat. Nos. 5,492,627 (Hagen, et al.); U.S. Pat. No. 5,558,771 (Hagen, et al.); U.S. Pat. No. 4,364,775 (Starkovich); U.S. Pat. No. 4,094,669 (Balko, et al.); U.S. Pat. No. 4,950,408 (Duisters, et al.); U.S. Pat. No. 5,209,773 (Audeh, et al.); U.S. Pat. No. 5,173,286 (Audeh) and U.S. Pat. No. 5,437,797 (Helmig). Briefly considering these patents, the Hagen, et al. patents disclose a porous support and web that filters out the mercury (elemental, ionic, or organic) from a fluid stream (column 2, lines 10-11). The porous web can be made from a polyamide, polytetrafluoroethylene, or a polyolefin (column 2, line 24-26). The support is coated with gold or a tin-salt which amalgamates the mercury and thus separates the mercury out from the fluid. The Starkovich patent discloses a process that removes mercury from a solid substrate by exposing the substrate to an oxidant containing compound (column 3, line 12). Examples of oxidants include nitric acid and potassium permanganate (see table I). The Balko, et al. patent discloses the filtering of mercury by first precipitating the mercury as mercury sulfide, then reacting the mercury sulfide with an oxidant and reducing the mercury to a metallic state with a reducing agent. The metallic mercury is then collected on a filter (column 1, lines 55-56). The Duisters, et al. patent discloses, in reference to the prior art, a process for mercury removal using an oxidizing agent (column 1, line 47). The Duisters patent removes mercury by using an active thiol-group (e.g., di-thiocarbamic acid) to absorb ionic (oxidized) mercury. The Audeh ('286) patent discloses a trap for trapping elemental mercury in a source deposit so that the mercury does not leak into the environment. Mercury-containing deposits are treated with inorganic sulfur compounds such as sulfides, alkali metal thiosulfates, and alkali metal which convert the soluble mercury to insoluble mercury and keep the mercury from "leaking" out of the deposit (column 1, lines 50-58). The Audeh ('773) patent discloses a guardbed that removes mercury from a gaseous stream. The guardbed contains two porous substrates that are coated with a mercury amalgamable material such as gold, silver or mixtures thereof (column 4, line 20). Any mercury that passes over this material amalgamates on the substrate and is removed. The Helmig patent discloses a process where organic and inorganic mercury is removed by passing a mercury containing aqueous stream through a macroporous cross-linked polystyrene chelating resin containing a polyisothiouronium functional group (column 4, lines 35-39).